But this time, the climate anomaly that usually brings warmer and drier winters to the Pacific Northwest might not have such a noticeable impact, say two University of Washington climatologists.

The reason is that the Pacific decadal oscillation, or PDO, a decades-long climate shift in the northern Pacific Ocean, appears to have moved into its cool phase since the last El Niño in 1997-98.

“If that’s the case, there is a distinct possibility that the effects of El Niño will be reduced this time around, at least for the region’s temperature” said Nathan Mantua, a research scientist with the Pacific Northwest Climate Impacts Group at the UW. “The confidence in a typical El Niño forecast for a mild, dry winter in the Northwest should be tempered by the potential influence of the PDO.”

The presence of El Niño “tilts the odds towards mild and dry, but the PDO tilts the odds the other way,” said Philip Mote, a research scientist with the Climate Impacts Group.

“This combination of warm El Niño and cool PDO usually gives us near-normal temperatures and below-normal precipitation,” he said.

In recent decades, El Niño has emerged as perhaps the most-powerful and best-known pattern of climate variability, recurring every four or five years. But overlaying that variability, scientists have found, is the much-longer PDO cycle, with positive and negative phases lasting anywhere from 10 to 30 years each.

Last week, the National Oceanic and Atmospheric Administration’s Climate
Prediction Center confirmed that El Niño has returned to the tropical Pacific Ocean. The center said this El Niño event would be milder than the last one four years ago, and that its effects would start to be felt this fall.

Scientists for months have been tracking increasing sea-surface temperatures in the tropical Pacific and have expected El Niño to re-emerge. The warmer-than-normal equatorial water results in rising wet air that can modify winds. That in turn can trigger weather changes around the globe, typically causing warmer, rainier winter weather in the southern United States, for instance, and drier weather in much of Indonesia.

El Niños and their opposites, La Niñas, tend to occur every four or five years. They can last 12 to 18 months, and in between episodes the Pacific usually returns to normal temperatures.

The last time El Niño was tempered by the PDO was in the winter of 1972-73. That winter saw near-normal temperatures and below-normal precipitation, typical results if an El Niño occurs while the PDO is in its cool phase. Since 1977, when the PDO shifted to its positive, or warmer, phase, El Niño events have only occurred with a positive PDO, a combination that could strengthen El Niño’s impact on the United States.

“The PDO has been a spoiler in the past when we’ve been in this configuration going into the fall,” Mote said.

Until now, that is. That’s because in 1998 the PDO appears to have shifted back to its negative, or cool, phase and this year is the first time that shift coincides with El Niño conditions, said Mantua.

The effects of the Pacific decadal oscillation are most pronounced in Alaska, western Canada and the very northern tier of western states, so those areas are least likely to see the full effects of El Niño.
While the ocean patterns are helpful for seasonal forecasting, Mote noted that their success goes only so far.

“In 2000-2001, the ocean was pointing toward a cool, wet winter in the Northwest and we had the worst drought since 1977,” he said.

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For more information, contact Mantua at (206) 616-5347 or mantua@atmos.washington.edu; or Mote at (206) 616-5346 or philip@atmos.washington.edu.